I. Field of the Invention
The present invention relates to wireless communications. More particularly, the present invention relates to a method and system incorporating the use of interference cancellation during the forward link portion of a wireless telecommunication system communication.
II. Description of the Related Art
FIG. 1 is a illustration of the radio frequency (RF) electromagnetic signal transmissions associated with the forward link portion of a code division multiple access (CDMA) wireless telecommunications system. Base transceiver stations 100(a) and (b) transmit multiple access spread spectrum forward link signals 102(a) and (b) that are upconverted to an RF bandwidth. Via the process of reflection, building 106 generates forward link signal 102(c) in response to forward link signal 102(a). Subscriber unit 104(a) is positioned such that it receives forward link signals 100(a), (b) and (c), while subscriber unit 104(b) is positioned such that it receives only forward link signals 102(a) and (b). In an alternative configuration, one or more of base transceiver stations 100 generate multiple forward link signals 102 that are transmitted to portions of the surrounding area, usually referred to as "sectors", via directional antenna systems.
Each forward link signal 102 is comprised of a set of channels each of which carries one kind of information necessary to conduct the forward link portion of any communication (normally a telephone call) with subscriber units 104(a) and (b). The various kinds of information include pilot data for detecting the presence of a forward link signals 102, synchronization data for synchronizing with forward link signal 102, paging data for notifying a subscriber unit 104 of an incoming communication, and various sets of traffic data which generally consist of digital audio information, digital data, or both. The digital audio information is usually an electronic representation of the physical sound waves associated with actual audio information, preferably generated via the process adjusting the voltage level of a node within a telephone or other electronic system based on samples of the sound waves. The electronic information is digitized by performing periodic sampling of that voltage, and by generating binary numbers corresponding to the voltage potential detected at each sample. Various techniques for encoding and compressing these binary numbers well known in the art may also be employed. Upon receiving each forward link signal 102, subscriber units 104(a) and (b) separate the channels they need for their particular communication from the remaining channels via various type of signal processing. The remaining channels contain the information used to conduct other communications with other subscriber units 104 in the same area (not shown) which are also carried by each forward link signal 102.
The generation of the set of channels necessary for transmitting the multiple kinds of information via a single forward link signal 102 is performed via the use of a set of channel codes each of which is orthogonal to the remaining set. Before transmission, each bit of data associated with each kind of information is direct sequence modulated in a synchronized fashion with one of the channel codes from the set of channel codes. In one implementation of such a system, sixty-four channel codes are used, each channel code containing sixty-four chips with each chip having a value or either 1 or -1, with a -1 being used to represent a logic 1 and a 1 representing a logic 0. Once modulated, the various types of data are spread via direct sequence modulation with a common spreading code which is also comprised of a series of 1 and -1 values. The spreading code is generally much longer than the channel codes and only a portion of it is applied to any particular bit of data. The spread data is then summed together and upconverted for transmission via forward link signals 102. As shown in FIG. 1, multiple instances of these forward link signals are generated either by separate base transceiver stations 100, or via the process of reflection. Each of the forward link signals can then be received by subscriber units 104(a) and (b).
Upon receiving a set of multiple forward link signals 102, subscriber units 104(a) and (b) despread and demodulate a sub-set of these forward link signals in order to separate out the data necessary to conduct a communication. The sub-set is selected on the basis of signal quality and which forward link signals 102 promote diversity of signal source. If fewer than a certain number of forward link signals 102 are received, all of the forward link signals 102 can be demodulated. The demodulation is performed with a particular channel code from the set of orthogonal channel codes that has been assigned to the desired data. The demodulation of the forward link signal 102 with a particular channel code removes other orthogonal energy from that forward link signal 102, thereby isolating the desired data associated with that channel code from the remaining data so long as the set of channels within each forward link signal remain synchronized.
FIG. 2 is a block diagram of the RF signal reception and processing portion of a subscriber unit 104 (FIG. 1) when configured in accordance with the prior art. During operation, any RF signals received by antenna system 202 having frequencies that fall within a predetermined bandwidth are downconverted by RF signal processing system 203 and supplied to AGC system 200. AGC system 200 measures the energy level of the downconverted signals and amplifies or attenuates them as necessary to place the energy level of those signals within a predetermined decibel range. The gain adjusted signals are then applied to analog signal processing system 201 which further downconverts and digitizes the signals, and applies the digitized signals to searcher 206. Searcher 206 receives the digitized signals and identifies any forward link signals 102 transmitted from base transceiver station 100 by searching at various time offsets for the associated pilot channel.
When a forward link signal 102 is detected, searcher 206 calculates an arrival time for that forward link signal 102, which in the preferred embodiment takes the form of a time offset from a synchronization signal, and provides that information to control system 205. Control system 205 then assigns one of despreaders 208(a)-(c) to despread the forward link signal 102 using the time offset. Despreading is generally performed via direct sequence demodulation, which in one implementation constitutes performing a chip-by-chip multiplication operation on the data using the same spreading code used to spread the data originally. As additional forward link signals 102 are detected, control system 205 identifies those of the highest quality and assigns despreaders 208(a)-(c) to despread those signals.
The resulting despread signals from despreaders 208(a)-(c) are passed to traffic channel demodulators 210(a)-(c) which demodulate the signals using a channel code associated with the desired traffic data, with the appropriate channel code being unique for each subscriber unit 104 engaged in a communication with a particular base transceiver station 100. In one implementation of such a system, the demodulation with the channel code comprises performing a chip-by-chip multiplication operation with the data using the entire channel code and then summing the results of the multiplications to obtain an estimate of the data being transmitted. The estimates from traffic channel demodulators 210(a)-(c) can then be received from nodes 212(a)-(c) by other signal processing systems within a subscriber unit 100 (not shown), which will generally combine the estimates using various well known techniques in order to generate a single estimate of the data used for further processing.
During the processing of a particular forward link signal 102, the channels within that forward link signal 102 remain synchronized because they are transmitted via a single RF signal, and therefore travel the same path to arrive at a particular destination such as subscriber unit 104. This is not the case for channels carried by different forward link signals 102, however, because different forward link signals 102 generally travel different paths and therefore different distances to arrive at a particular destination. These different distances causes each forward link signal 102 to arrive with a slight time offset with respect to other forward link signals 102 which removes any orthogonality between the channels carried by one forward link signal 102 with respect to channels carried by another forward link signal 102. This lack of orthogonality prevents the energy associated with the channels of a first forward link signal 102 from being removed completely from a second forward link signal 102 via demodulation with a channel code. While the presence of this unremoved energy degrades the quality of any data produced using the second forward link signal 102, the degradation is generally not to a degree sufficient to prevent proper operation of the wireless telephone system. Nonetheless, if a method for processing a forward link signal could be developed that allowed at least some of the non-orthogonal energy from the other forward link signals 102 to be removed, such a development would substantially improve the quality of the data produced by a subscriber unit 104 incorporating the use of that method. This improved quality would also result in a reduction in the amount of power necessary to complete the transmission of data, which in the context of a CDMA wireless telecommunications system allows for increased data carrying capacity. Therefore, such a development would be highly desirable.